1. Field of the Invention
This invention is directed to a network analyzer having one or two test ports, and a method of calibrating such a network analyzer.
2. Description of the Prior Art
In the field of radio-frequency engineering, network analyzers are used to measure scattering parameters by means of which components or networks are described. There exist network analyzers having two test ports each of which is in most cases linked to two measuring points so that four measured values are respectively obtained from a test object linked between the two test ports. A network analyzer of this type is described, for instance, in U.S. Pat. No. 4,982,164. There are also so-called unidirectional network analyzers which also comprise two test ports one of which is linked to two measuring points while the other one is linked to only one measuring point. Finally, there are also network analyzers which operate as reflectometers and which include only a single test port linked to two measuring points.
All of these network analyzers need to be calibrated prior to the actual testing of objects. To this end there are various possibilities which have been described for example by
EUL, H.-J., SHIEK, B., A generalized Theory and New Calibration Procedures for Network Analyzer Self-Calibration, IEEE Trans. on Microwave Theory and Techniques, vol. MTT-39, April 1991, pp.724-731, or PA0 EUL, B.-J., SHIEK, B., Reducing the Number of Calibration Standards for Network Analyzer Calibration, IEEE Trans. on Instrumentation and Measurement, vol. MTT-40, August 1991, pp.732-735 or, respectively, in PA0 German Patent Specification 3,912,795 or U.S. Pat. No. 4,982,164. A particularly simple calibration method of the specified kind requires only three calibration standards which can readily be realized (U.S. Pat. No. 4,982,164, hereby incorporated by reference).
With the so-called TMS or TMR calibration the test ports are initially through-connected (Through), then a calibration is performed with an attenuator which is linked between the test ports and matched to be reflection-free (Match), and finally a calibration is made with a short of the two test ports (short Circuit) or with a reflection termination (Reflection). With the so-called LMS technique, a line (Line) is initially linked between the test ports as a first calibration standard, and calibration is then continued with an attenuator and a short.
During calibration the transmission and reflection parameters of these calibration standards are measured and error parameters are determined therefrom which are then stored in a memory of the evaluation means of the network analyzer and will be taken into account for error correction in the subsequent object measurement.
It is not possible, however, to take into account any variations of the electrical properties due to temperature fluctuations or the like which occur immediately after calibration so that these variations are directly introduced as errors into the measuring results. It is the demand for accuracy made by the user of such a network analyzer in respect of the test results which determines the time interval between the calibration operations. This interval may be one or two days or may only be in the range of a few hours. In some cases the calibration efforts including the connection of the discrete calibration standards to the test ports are quite considerable. In the field of industrial measurement engineering a repeated calibration is frequently quite difficult and incurs high costs because sometimes the running production must be stopped or the analyzer setup must even be removed.